1. Field of the Invention
The present invention relates generally to assay methods, and more particularly to solid phase membrane assays employing visual labels.
2. Description of the Background Art
A wide variety of immunoassay methods have been developed for detecting and quantitating numerous analytes in liquid samples. Such assays may generally be classified in the following categories: competitive or non-competitive, homogeneous (liquid phase) or non-homogeneous (solid phase), and according to label such as radioactive or visual label. The present invention is particularly concerned with methods and kits for performing non-homogeneous (solid phase) assays employing visual labels, particularly color labels, by both competitive and non-competitive techniques.
A large number of assays have been developed even within the limited category just described. For example, U.S. Pat. Nos. 3,888,629; 4,366,241; and 4,632,901, each describe assays employing a membrane immunoadsorbent in combination with an absorbent pad. Liquid sample is applied to the pad by various techniques, and the sample drawn through the entire membrane area by capillary action of the absorbent pad. The membrane is exposed to labelled antibody, and binding of the labelled antibody to the membrane is proportional to the amount of analyte in the sample. Assays of this type are particularly convenient for performance outside of clinical laboratories as they require few steps, short incubation times, and the materials are readily disposable (at least if non-radioactive labels are employed). Heretofore, however, such assays have been generally unsuitable for samples characterized by very low analyte concentrations and/or very low sample volumes where the analyte is bound to the membrane at very low levels. Frequently, the signal observed on the membrane pad is so weak that it is impossible to tell whether analyte was present in the sample.
It would therefore be desirable to provide methods and kits for performing membrane assays having improved sensitivity and readability. It would be particularly desirable if such methods and kits retained the convenience, economy and rapid performance characteristic of previous membrane assays.